1. Field of the Invention
This invention relates to an injection molding machine for molding a product having a desired profile by injecting molten metal into a metal mold by means of a plunger, and an injection molding method for molding such a product.
2. Prior Art
FIGS. 8A and 8B of the accompanying drawings illustrate a typical known metal molding machine comprising a melting pot (not shown) for storing molten metal 10' and a plunger 12' having a lower portion dipped into the molten metal 10' in the melting pot, wherein the molten metal 10' is fed from the plunger 12' to an injection nozzle 14' and then injected into a metal mold 16'. The plunger 12' has a cylinder section 18' dipped into the molten metal 10' and a piston 20' that slidingly reciprocates within the cylinder section 18', which cylinder section 18' has a through hole 22' bored through an upper portion of its lateral wall, through which the molten metal 10' is drawn into the cylinder section 18'. A rod 24' is linked to the piston 20' at an end thereof and to a corresponding rod 28' of another piston 30' housed in a hydraulic cylinder 26' at the other end.
The illustrated known injection molding machine operates in a manner as described below. Firstly as shown in FIG. 8A, the piston 30' of the hydraulic cylinder 26' is raised to pull up the piston 20' of the plunger 12' above the through hole 22' by way of the rods 24' and 28'. Since the through hole 22' is located below the surface L' of the molten metal 10', the molten metal 10' flows into the cylinder section 18'. Subsequently, the piston 30' of the hydraulic cylinder 26' is moved downward to lower the piston 20' of the plunger 12', and after the piston 20' passes the through hole 22', the molten metal 10' is forced in the cylinder section 18' to move out toward the nozzle 14' until it is injected into a cavity of the metal mold 16' through the outlet of the nozzle 14'. When the metal in the metal mold 16' is solidified, the metal mold 16' is opened to take out the molded product. Thereafter the piston 30' of the hydraulic cylinder 26' is raised to pull up the piston 20' of the plunger 12' as shown in FIG. 8B. Then, the molten metal 10' in the cylinder section 18' moves back from the nozzle 14' as the piston 20' is pulled up, and once the piston 20' passes by the through hole 22', falls until its surface is flush with the level L' of the surface of the molten metal 10' in the melting pot because the inside of the plunger 12' is subjected to the atmospheric pressure.
With the above described prior art metal mold machine, the molten metal 10' in the nozzle 14' is forced to move back and forth in each injection molding cycle between the nozzle tip and the level L' of the molten metal in the melting pot to cover a relatively long traveling distance so that the molten metal 10' can take in bubbles to a large extent at the time of injection. Additionally, a long traveling distance means a long injection molding cycle time and a low efficiency.
There has been proposed an injection molding machine, as shown in FIGS. 9A and 9B, that provides an improvement to the above described known machine by arranging, in addition to the features of the machine of FIGS. 8A and 8B, a non-return valve 34' at the bottom of the cylinder section 18' so that molten metal 10' can flow from the melting pot into the cylinder section 18' but cannot flow back from the cylinder section 18' and hence the molten metal 10' is inevitably moved through the injection path 15' and injected through the nozzle 14'.
However, with this arrangement, the molten metal 10' can remain in the injection path 15' all the way to the tip of the nozzle 14' to deter the initial rate of injecting the molten metal in the next injection molding cycle so that the molded products may show flaws on the surface.
In view of the above identified technological problems, it is therefore an object of the present invention to provide an injection molding machine having a simple structure and adapted to operate for injection molding with a short cycle time so that the molded products show an improved surface. Another object of the invention is to provide an injection molding process using such a machine.